Electric vehicles, hybrid electric vehicles (HEV's), plug-in hybrid electric vehicles (PHEV's) and other vehicles that utilize an electric machine, such as an electric motor, may be configured to use the electric machine to provide regenerative braking to at least assist in stopping the vehicle. Regenerative braking may provide a number of advantages over using a friction braking system exclusively. For example, the use of regenerative braking, whereby an electric motor provides negative torque to the vehicle wheels, reduces wear on the friction elements of the friction braking system. In addition, during regenerative braking, the motor may function as a generator, producing electricity that may be used immediately or stored in a storage device, such as a battery.
With a step-ratio transmission in a vehicle, there may be a torque hole during the ratio change of a downshift due to the change in inertia from a higher gear to a lower gear. In a conventional powertrain, this torque hole can be filled by reserving some engine torque via spark at the beginning of the shift, and then releasing the torque during the ratio change to fill-in for the inertia torque. In an HEV or PHEV powertrain, the downshift is typically executed during regenerative braking when the engine is off, so this solution may not be available. In at least some HEV and PHEV powertrains, the high voltage motor torque is operating with very negative torque—i.e., it is being used to charge an electrical storage device such as a battery—so to fill the torque hole during the shift, it is necessary to increase motor torque from a low negative value toward zero. This action reduces the amount of regenerative braking that can be captured, and reduces fuel economy. Additionally, it is not always possible to use the motor to fill in the torque hole. There are times when the motor has low headroom, such as a high battery state-of-charge (SOC) condition where the battery is full and cannot accept any more charge, or if the motor or battery is hot and the limits of electrical system have been reduced in an effort to cool them down. In these situations, the motor may not have enough capability to counter the inertia change in the shift, leading to inconsistent shift operation.